This invention relates, in general, to field effect transistor (FET) distributed amplifiers, and more specifically, to FET distributed amplifiers with high gain and broad band widths.
Distributed amplifiers incorporate a series of amplifiers coupled in parallel to an input transmission line. Each of the amplifiers receives a portion of an RF (radio frequency) signal, or other type of signal, and amplify that particular portion of signal. The amplified signals output by the respective amplifiers are then combined to yield an output with high gain and high power output with respect to power input. A problem inherent within these distributed amplifiers is a loss of approximately 3 dB (decibals) through terminating what is known as unguided signals. When the respective amplifiers of the distributed amplifier output the amplified signals to the output transmission line, the amplified signals are split with part transmitted to the output (guided portion), and the other part, or unguided signals, terminated. Termination of the unguided signals results in the 3 dB loss.
Gain of a distributed amplifier can be approximated by the following equation: EQU Gain(G)=20 Log(N gm Z.sub.o)/2
where
N=number of distributed amplifier devices, PA1 gm=transconductance, PA1 Z.sub.o =transmission line impedence, and
G is bounded by the maximum device gain. Since increasing N results in diminishing returns on power (due to increased line losses), G can effectively be increased only by increasing gm. Many networks designed to increase G have increased N with partial success, but such devices become unuseable after the number of active devices exceed three for the reasons explained above. Furthermore, increasing the number of devices increases the cost of the amplifier since the size of the amplifier increases with each device added. This is particularly a problem with amplifiers designed for MMIC (monolithic microwave integrated circuits) technology where size is to be minimized to increase yields. Some designs have attempted to increase gm, but have achieved only partial success.